Document DE-C-291 548 discloses a type of means for directly fixing a bridge to the edge of a mineral lens.
Another category of frames of the aforesaid type, known as "three-piece frames", is available commercially.
These are frames which do not include a part adapted so as to encircle the periphery of the lenses, and which have therefore been designed so as to meet a desire for lightness and discreetness. The bridge and the side-arms of these frames are fixed to the lenses by screwing through the lenses which have therefore to be drilled for this purpose. Screwing requires recourse to a set of screws, washers, nuts and lock-nuts.
Such frames nevertheless have several major drawbacks, resulting in particular in their not being able to be assembled with lenses other than by a highly experienced and highly skilful optician. As a matter of fact, the choice of the angle at which the drilling is to be performed is critical in order that, once assembly has been carried out, the lenses are properly located in the desired wearing plane; once the angle has been chosen, it is difficult to make it comply with the drilling; finally, the optician must take all precautions to prevent the lens from cracking either during the initial drilling, or subsequently, namely during the operation of tightening the screws after fitting, or even in the course of use, under the effect of the stress exerted on the lens. In the event of breakage, the optician has to fashion a new lens and again devote considerable time and effort to the fixing thereof, and this may give rise to very significant costs. As a result, numerous opticians are reticent to propose such frames. Let us add that these frames have, for the optician, the drawback of entailing a relatively complex and costly assembly device, owing to the diversity of the pieces forming the fixing means.
Coming to the wearer, the placement of the fixing means generates a certain occular nuisance, in particular in the nasal area, owing to the fact that the screws are located within his field of vision. In extreme cases, the wearer may even manifest a convergent squint (wearers exhibiting small pupillary distances).
Finally, spectacles which include such frames are tricky to maintain. Apart from their intrinsic fragility, a problem results from the fact that, in such frames, the screws serving to fix the bridge pass through the lens and through a tab appertaining to the bridge and which is thus applied closely against the internal face of the lens. Dirt slips between this tab and the lens, which dirt is inaccessible but visible from the outside face of the lens. Ultrasound cleaning does not generally allow this dirt to be totally eliminated and the only means of removing it completely is to dismantle the bridge, with the risk of breaking the lens which this entails when dismantling and refitting.